DCE is usually prepared by oxychlorination of ethylene using hydrogen chloride (HCl) and a source of oxygen or by direct chlorination of ethylene using chlorine. The dehydrochlorination of DCE by pyrolysis thus results in the production of VC with release of HCl. The oxychlorination and chlorination are generally carried out in parallel and the HCl produced in the pyrolysis is used in the oxychlorination.
To date, ethylene which is more than 99.8% pure is normally used for the manufacture of DCE. This very high purity ethylene is obtained via the thermal cracking of various petroleum products, followed by numerous complex and expensive separation operations in order to isolate the ethylene from the other products of the cracking and to obtain a product of very high purity.
Given the high cost linked to the production of ethylene of such high purity, and also the advantage that there could be in envisaging a process for the manufacture of VC by DCE in favourable regions that lack accessible ethylene capacities, various processes for the manufacture of DCE using ethylene having a purity of less than 99.8% have been envisaged. These processes have the advantage of reducing the costs by simplifying the course of separating the products resulting from cracking of petroleum products and by thus abandoning complex separations which are of no benefit for the manufacture of DCE.
Thus, various processes for the manufacture of DCE starting from ethylene having a purity of less than 99.8% produced by simplified cracking of ethane have been envisaged.
For example, Patent Application WO 00/26164 describes a process for the manufacture of DCE by chlorination of ethylene obtained by simplified cracking of ethane, the chlorination taking place in the presence of impurities obtained during the cracking of ethane without any other purification.
Patent Application WO 03/48088 itself describes a process for the manufacture of DCE by dehydrogenation of ethane giving rise to the formation of a fraction comprising ethane, ethylene and impurities including hydrogen, which fraction is then subjected to a chlorination and/or oxychlorination.
These processes have the disadvantage that the ethylene obtained cannot be used for a combined ethylene chlorination/oxychlorination process given that the ethylene contains impurities whose presence during the oxychlorination reaction could cause operating problems, namely poisoning of the catalyst by the heavy products and an uneconomic conversion of the hydrogen present. This hydrogen conversion would consume high-purity oxygen which would thus be sacrificed for an undesired reaction and would release a high heat of reaction during the conversion of hydrogen to water. This conversion would then limit the capability of the oxychlorination reactor, generally linked to the heat exchange capability. An unusually high investment must therefore be expended in order to guarantee the heat exchange area, and thereby the reactor volume, caused by the presence of hydrogen in the mixture.
The option taken of burning the hydrogen in a separate reactor, described in Application WO 03/48088, does not resolve the difficulty because it requires a large amount of oxygen, a stoichiometric amount relative to hydrogen, and also a large surface area for exchange to eliminate this heat of combustion. Consequently it has a significant ethylene consumption and it may have problems linked to safety. Finally, the removal of the water formed leads to an increase in the production costs.
Processes in which VC is obtained by oxychlorination of ethane and not of ethylene are also known. Such processes have not found an industrial application up till now given that as they are conducted at high temperatures, they result in a mediocre selectivity with loss of the reactants used and costs for separating and destroying the by-products and they are also characterized by problems of behaviour of the materials in a corrosive oxychlorination medium. Finally, problems linked to the behaviour of the catalysts used owing to the gradual vaporization of their constituents and also linked to the deposition of these constituents on the cold surface of the exchanger bundle are usually encountered.